Sensory constraints in temporal segregation in two species of anglerfish, Lophius budegassa and L. piscatorius

The temporal segregation of 2 ecologically equivalent and sympatric species, the black anglerfish Lophius budegassa and white anglerfish L. piscatorius, was studied in relation to day-night behaviour and the underlying morphological variation in the sensory organs. Temporally scheduled trawl sampling was conducted on the western Mediterranean shelf, and diel fluctuations in catches were used as a proxy of behavioural rhythms. L. budegassa was more abundant at night than L. piscatorius, which mostly occurred in daytime catches. The observed differences in the timing of maximum activity (i.e. catches) were consistent with differences in otolith sagittae weight, eye/lens sizes and optical sensitivity. L. budegassa presented significantly heavier otolith sagittae than L. piscatorius, indicating that the behaviour of the black anglerfish is mostly acoustically driven (a typical characteristic of species living in dim light conditions). Preference for a dim light environment was also confirmed by the high optical sensitivity observed in this species achieved by enlarging the photo-receptors' diameter. In contrast, L. piscatorius showed larger eyes and lenses than L. budegassa, but only in individuals larger than 50 cm. Large eyes usually improve photon capture and the fineness with which the image is sampled; however, relatively low values of sensitivity and resolution were observed in large L. piscatorius. This indicates that a trade-off might exist between the increase in eye size as an adaptive response to the impending deep meso-and bathypelagic life and a visual constraint imposed by the quality of the water at the bottom level where contrast can be degraded by suspended particles. These results indicate that interspecific competition with sympatric and ecologically equivalent species may result in different adaptations to diurnal/nocturnal behaviour, which are sustained by morphological specialisations in sensory organs.